1. Field of the Invention
The present invention relates to a wireless CDMA system, and more particularly, a wireless CDMA system having a unique forward control channel dedicated for transmission of system overhead information.
2. Description of Related Art
Conventional CDMA systems employ a plurality of channels on a single CDMA carrier. The system differentiates between the channels by assigning a different Walsh code to each channel. Specifically, each base station in the system transmits a reference signal called a pilot channel having a Walsh code of 0. The pilot channel generated by one base station for a particular CDMA carrier generally is the same as the pilot channel generated by another base station for the same CDMA carrier except for being shifted in time. The other channels associated with a pilot channel have the same time shift, but different Walsh codes from the pilot channel.
For instance in one CDMA standard, a sync channel is Walsh code 32. Having detected the strongest pilot channel, a mobile (e.g., a cellular phone) accesses the associated sync channel using the Walsh code 32. The sync channel includes information on the base station and communication capabilities thereof such as a pilot offset index, a long code state, a system time, etc.
The pilot offset index indicates the time shift of the pilot channel generated by the base station. The long code indicates the state of the long code for accessing a forward control channel at the time given by the system time. Forward control channels having the format shown in FIG. 1 are traditionally referred to as paging channels, and will be referred to as such in this description. Using the long code state, the mobile accesses the first of a possible plurality of paging channels referred to as the primary paging channel.
As shown in FIG. 1, each paging channel is divided into a plurality of slots. Overhead message slots 10 alternate with mobile specific message slots 12. Mobile specific message slots 12 generally include a message for a particular mobile communicating with the base station. An overhead message slot 10 includes overhead messages for all mobiles communicating with the base station. Because the base station can only service a finite number of mobiles using one paging channel, a base station may employ more than one paging channel.
Besides the pilot channel, sync channel, and paging channel(s) discussed above, the typical wireless CDMA system further includes a plurality of traffic channels and an access channel. It is over the access channel that mobiles send messages to the base station when the mobile is not on a traffic channel, and it is on a traffic channel that communication between the mobile and base station takes place. For example, via the access channel, the mobile will request that the base station assign a traffic channel for the mobile to use in making a call.
The overhead messages in the overhead message slots 10 of each paging channel generally include at least the following messages: a system parameters message, a neighbor list message, a channel list message, and an access parameters message. It should be understood, that generic names have been given to each of these messages, and that the particular name is not intended to reference a particular CDMA standard. Instead, each message is intended to cover the message or messages in a particular CDMA standard that meet the definition given below for each message. Furthermore, each message will only be described to the extent needed to better understand the present invention.
The system parameters message informs a mobile of what parameters to use in tasks such as pilot channel searching, hand-off, power control, etc. For example, the hand-off parameter gives a signal level threshold which a mobile uses in deciding whether to switch base stations. Namely, if a detected pilot channel of a base station not currently servicing the communication needs of a mobile exceeds the signal level threshold, the mobile will switch to this base station.
The neighbor list message indicates the pilot offset index and configuration structure for the base stations of neighboring cells. The configuration structure information indicates whether the neighboring base station is configured the same as the current base station, and/or any differences in that configuration (e.g., location of paging channels). The channel list message indicates the number of paging channels the base station is using. The access parameters message informs the mobiles of the communication format that the mobile should use in accessing the base station.
Besides the information discussed above, the system parameters message, the neighbor list message, and the channel list message include the same common message sequence number; also referred to as the configuration sequence message (CSM) number. The CSM number changes if any parameter in the system parameters message, the neighbor list message, or the channel list message changes. By comparing the CSM number of overhead message information a mobile currently stores with the CSM number in a detected overhead message, a mobile decides whether to update the stored overhead message parameters from the system parameters message, the neighbor list message, and the channel list message. Similarly, the access parameters message further includes an access message sequence (AMS) number. The AMS number changes if any parameter in the access parameters message changes. By comparing the AMS number of overhead message information a mobile currently stores with the AMS number in a detected overhead message, a mobile decides whether to update the stored overhead message parameters of the access parameters message.
In addition to the overhead messages and mobile specific message discussed above, the base station also generates a general page message. This message indicates, among other things, the current CSM and AMS numbers. A mobile specific slot 12 can include only mobile specific messages, only general page messages, or both mobile specific messages and general page messages.
When a mobile is first turned on, the mobile performs an initialization process such as shown in FIG. 2. First, in step S10, the mobile detects the strongest pilot channel, and then camps onto the associated sync channel in step S15. Using the information on the sync channel, the mobile camps onto the primary paging channel (generally the paging channel having the lowest Walsh code) in step S20, and obtains and stores the parameters in the channel list message from the overhead messages in step S25. As discussed above, the channel list message includes information on the number of paging channels employed by the base station.
Then, in step S30, the mobile determines from the parameters in the channel list message if the base station transmits more than one paging channel. If so, the mobile determines in step S35 the particular paging channel to monitor. This is accomplished using a well-known hashing algorithm which computes the paging channel number based on the number of possible paging channels and the identification number of the mobile. The mobile identification number and hashing also indicates which slot of the computed paging channel the mobile should monitor for mobile specific messages. More than one mobile can map to the same mobile specific slot of a paging channel. Mobiles differentiate between the messages destined for them and destined for other mobiles by recognizing their identification number in the mobile specific message.
Next, in step S40, the mobile hashes onto the paging channel determined in step S35, and obtains the information in the overhead messages (hereinafter collectively referred to as xe2x80x9cthe overhead parametersxe2x80x9d) from this paging channel in step S45. The mobile then continuously determines whether to update the stored overhead parameters in step S50 based on the CSM and AMS numbers in the manner discussed above.
In step S30, if the mobile determines that only one paging channel exists, then the mobile obtains the overhead parameters in the overhead messages in step S55 from the primary paging channel, and determines the mobile specific slot to monitor in step S60 in the same manner discussed above with respect to step S35.
After the initialization process is complete, the mobile may enter an idle state wherein the mobile is not engaged in communication with the base station. During the idle state, the mobile may monitor particular slots such as its mobile specific slot, and sleep the rest of the time to save battery life. This mode of operation is referred to as the slotted mode. The mobile may also operate in an unslotted mode wherein the mobile monitors all slots. The unslotted mode is entered when the mobile operates to access the system.
If at any time the mobile loses the pilot channel, the initialization process must be repeated. The initialization process, beginning with step S15, is also repeated anytime the mobile switches base stations, referred to as an idle hand-off. The initialization process must be completed before a call can be made or received by the mobile. Additionally, before a call can be made, the mobile has to confirm its stored overhead parameters are current.
It takes a mobile on average about 600 ms to determine whether an update of the overhead parameters is required, and then up to another 1.28 seconds to actually update these overhead parameters, if necessary. Unfortunately, during this time, the pilot channel can be lost, or the switch to a new pilot determined; such that initialization must be repeated. As a result, the user""s desired operation, like placing a call, is delayed. These time periods can be reduced by increasing the number of overhead message slots in a paging channel at the expense of reducing the number of mobile specific message slots, but the mobile specific message slots cannot be completely eliminated. Furthermore, reducing the number of mobile specific message slots degrades system performance.
It has been determined that the shorter the initialization and/or overhead parameter update time, the greater the likelihood that, for example, a call will succeed.
The wireless CDMA system according to the present invention uses one of the forward control channels as a configuration channel. The plurality of message slots in this configuration channel only include overhead messages. The other non-configuration forward control channels traditionally used as paging channels, by contrast, do not include slots with overhead messages. Furthermore, besides the offset indices for neighboring base stations, the neighbor list message in the overhead messages indicates whether a neighboring base station supports a configuration channel and the location of a supported configuration channel. By using the configuration channel, a mobile significantly reduces the initialization and overhead parameters update time compared to mobiles in a conventional wireless CDMA system.